1. Field of the Invention
The invention relates generally to electrical control systems, and more particularly to systems and methods for controlling variable speed drives of the type used in connection with downhole oil production equipment.
2. Related Art
Crude oil is typically produced by drilling wells into oil reservoirs and the pumping the oil out of the reservoirs through the wells. Often, the oil is pumped out of the wells using electric submersible pumps. Electrical power is provided to electrical drive systems at the surface of the wells and, in turn, these drive systems provide the required electrical power to the pumps.
Normally, a single power source, such as a generator, will be used to provide electrical power to the drives for pumps in many different wells. As a result of the loading on the power source by the different pumps, as well as various other factors, the power supplied to the pumps' drives may be subject to variations and even interruptions.
While the variations in the power being supplied to the pump drives may be relatively minor, and the interruptions relatively short, they may nevertheless be very disruptive with respect to the operation of the pumps, particularly when the pumps are submersible pumps operated in deep wells. The reason for this is that, since submersible pumps must fit in a well, they must be long and narrow. Consequently, they have very little inertia and, when there is a power variation/interruption, these pumps slow down and stop very quickly in comparison to pumps which have more inertia, such as surface pumps. The deceleration of the pump is even more pronounced in deep wells due to the large fluid column above the pump. Also, as the pump slows down, there will be some RPM below which the pressure produced by the pump is insufficient to support the column of fluid. Because of this, the fluid starts to fall back through the pump, which dramatically increases the torque required for continued forward rotation. This makes it necessary to stop the pump and wait for the column of fluid to drain back. The flowing of the fluid column back into the formation in some cases may take a few minutes, while in other cases it may take more than an hour. The delay in restarting the pump may be even longer because it may be necessary, if multiple pumps stop, to stagger the restart of each pump.
Because each well may normally produce hundreds or even thousands of barrels of oil in a day, the cost associated with a pump stopping and having to be restarted can be very high. There is therefore need to avoid such interruptions in production that are caused by variations and interruptions in the input power provided to the pumps' drives. Maintaining the operation of the pump during such interruptions is commonly referred to as “ride-through.”
Conventionally, the need for ride-through has been addressed by increasing the amount of capacitance already present in the DC bus and insuring that the power supply for the controls maintains a nominal operating voltage for the required amount of time. This can work if the load has enough inertia so that the speed does not fall below some recoverable RPM. However, the energy consumed from the DC bus during the event will need to be replaced when the power system recovers. With an uncontrolled converter section, this results in a current inrush sufficient to blow fuses or damage components. Some mitigation of this problem can be accomplished by adding impedance between the AC line and the input of the drive, but this only helps for relatively shallow line transients.
It would therefore be desirable to provide systems and methods for providing ride-through of interruptions in the power supplied to the pumps' drives, while avoiding the disadvantages of conventional ride-through mechanisms.